1. Field of the Invention
The invention relates to an agent for use in polymers as an antiblock agent. The invention further relates to a polymer composition containing an antiblock agent and a polymer film containing the antiblock agent.
2. Description of the Related Art
Various minerals have in the past been used to give antiblock properties to polyolefin films. The first large application for the use of polymeric films was in the food wrap industries, and for enclosing various materials in a transparent film for retail sales. Polyvinylchloride (PVC) resins were the first large volume materials used in these markets. PVC has a natural non-tacky property when produced as a film and does not require a material to provide an antiblock function. Such PVC films are still the material of choice for fresh food wrap, e.g., meat wrap, today. When polyethylene was developed, it was found to have more strength than and similar clarity to PVC when produced in a film. Polyethylene has, however, inherent tack. To address this problem and accelerate the use of polyethylene in packaging films and other thin-walled uses, e.g., bags, it was found that an antiblock or anti-tack agent (a mineral) would mechanically break up the film's natural smoothness, and thus reduce the ability of the film to stick to itself, i.e., the mineral agent acts as an antiblock agent.
Polyolefin films were originally made from low density polyethylene (LDPE) and high density polyethylene (HDPE) resins. As technology advanced, linear-low density polyethylene (LLDPE) was introduced for use in producing films. LLDPE has a higher strength than the prior materials, enabling thinner films to be produced for use in the same applications as LDPE. More recently, metallocene catalyst polyethylene resin systems have been developed for clearer films and more elastomeric properties than the prior polyethylene resins.
Polypropylene resins are also now used for films which require a very high degree of strength. Polypropylene has good clarity when produced as a film, but requires the use of expensive synthetic silica as an antiblock agent.
In the initial work with antiblock agents for polyethylene films, it was discovered that calcined diatomaceous earth of fine particle size (3.5 to 12 microns) worked very well as an antiblock agent. Calcined diatomaceous earth has continued to be used for this purpose for the last 35 to 40 years. Calcined diatomaceous earth contains a minimum of 63% crystalline silica and has irregularly shaped particles. The high levels of the crystalline silica in calcined diatomaceous earth have prompted a reduction of its use as an antiblock agent in polyolefin films from nearly 100% of the market to approximately 15-20% today, for reasons discussed in greater detail below.
Calcium carbonate has also been used as an antiblock agent for polyethylene films. Calcium carbonate has the advantage of being inexpensive. Calcium carbonate has the disadvantages of requiring an amount for effective antiblocking that is two to three times more than most other antiblock agents, which decreases the transparency of the resultant film. Films incorporating calcium carbonate as an antiblock agent vary in transparency from merely hazy to completely opaque. Calcium carbonate is therefore typically used as an antiblock agent for polyolefin films where clarity is not an issue, and opaque or colored films are acceptable. Calcium carbonate has irregularly shaped particles.
Microcrystalline silica has also been used as an antiblock agent. Silica had been used as an antiblock agent up until it was discovered that crystalline silica presents severe health hazards as a carcinogen. Microcrystalline silica has irregularly shaped particles and contains up to 98% crystalline silica. This ground sand or quartz also has high hardness which causes extreme metal wear in the equipment used to process the material.
In 1996, the World Health Organization--International Agency For Research on Cancer (IARC) issued a report (Silica and Some Silicates, IARC Monographs On The Evaluation Of The Carcinogenic Risk Of Chemicals To Humans, Vol. 42, 1997) identifying crystalline silica in respirable form as a Class 2A "probable human carcinogen." In late 1996, IARC announced the impending official change in that classification to Class 1, "human carcinogen." The designation of respirable crystalline silica as a human carcinogen presented severe problems with the use and incorporation of crystalline silica and crystalline silica-containing products, e.g., minerals, as antiblock agents in polyolefin films. Because Occupational Safety and Health Administration (OSHA) standards now severely restrict the amount of exposure that one can have to respirable crystalline silica, the use of antiblock agents which have high levels of respirable crystalline silica has been understandably greatly reduced.
Talc has also been used as an antiblock agent in polyolefin films. Talc has the benefit of requiring less material than calcium carbonate for the same level of antiblocking, and produces considerably less haze in the resultant film than calcium carbonate. However, the use of talc as an antiblock agent in polyolefin films requires special handling equipment to be able to process the talc for use as an antiblock agent, which increases overall costs. Talc has particles with a plate-shaped structure. Despite these failings, talc is now the most widely used mineral antiblock agent for LDPE and LLDPE films.
Nephylene syenite is also used as an antiblock agent in polyolefin films, despite its relative scarcity. Nephylene syenite includes no appreciable amount of quartz or crystalline silica, but has a very high hardness and is, consequently, extremely abrasive on processing and handling equipment. Nephylene syenite has a refractive index near that of polyethylene, which makes it very useful despite its relative scarcity. Currently, nephylene syenite is used in approximately ten percent of polyolefin film production applications.
Polymer producers add antiblock agents to the polymer in two different ways, near the end of the production chain. The most common way is to add the mineral and other additives "neat," i.e., mixing the antiblock agent in the appropriate percentage directly into the molten polymer as it is being produced.
A second method for adding antiblock to a polymer is to add a concentrate of the agent into the polymer. A concentrate is normally a blend of the antiblock agent and some polymer. Usually the concentrate ranges from 10% to 75% (by weight) antiblock agent, with the balance being polymer and other additives, if any. To prepare the concentrate, antiblock agent is added to an amount of the polymer so as to obtain the desired concentration of antiblock agent in polymer. The polymer is then extruded, cooled, and pelletized or granulated to form a concentrate.
Antiblock agents generally function in two ways. Antiblock agents may be present as a dusting on the surface of a film to prevent contact of the film to itself or other surfaces, thus preventing mutual adhesion. Examples of the use of an antiblock agent in such a manner include talc on surgical gloves. A second way of using an antiblock agent with a film is by intimately dispersing the agent in the polymer before the polymer is formed into the film. The antiblock agent particles extend beyond the outer surface of the polymer, thereby disturbing the planarity of the polymer film surface, thus preventing the surfaces of the film from fully contacting itself or another surface.
The amount that a polymer film blocks, i.e., that it adheres to itself or another surface, is quantifiable. One method of quantifying the blocking characteristics of a film is by measuring the coefficient of friction of the surface of the film using a standardized method. One such method is ASTM D 1894-95, Standard Test Method for Static and Kinetic Coefficients of Friction of Plastic Film and Sheeting, incorporated by reference herein in its entirety. Another method of quantifying the blocking characteristics of a film is by directly measuring the blocking load of a film. One such method is ASTM D 3354-89, Standard Test Method for Blocking Load of Plastic Film by Parallel Plate Method, incorporated by reference herein in its entirety.
In the past, pumice has been used as a filler in polymers. Typically, filler weight percentages in a bulk polymer are very high, resulting in extreme haze or opacity of the resultant polymer. For example, filler weight percentages range from about 10% to about 50%. Pumice is added to polymer matrices as a filler for a number of reasons, among them being to improve impact strength, tear strength, tensile strength, and increase stiffness. Furthermore, when used as a filler, pumice typically has a particle size of 200 microns or greater. For example, U.S. Pat. No. 5,536,773 (Yamada et al.), U.S. Pat. No. 5,492,741 (Akao et al.), U.S. Pat. No. 5,358,785 (Akao et al.), U.S. Pat. No. 5,262,288 (Kohyama et al.), and U.S. Pat. No. 4,124,550 (Kobayashi et al.) describe the use of pumice in polymers as a filler or as a light-shielding additive.
In contrast, minerals and other agents used as antiblock agents in polymer films are typically used in extremely small amounts, e.g., less than or equal to 1% of the polymer bulk material by weight.
Pumice has also in the past been used as an abrasive product, e.g., in abrasive soaps, etc., and polishing compounds. Pumice used in these applications is typically very coarse, includes particles over a very wide range of sizes, is used in extremely high concentrations or weight percentages, and varies considerably in both color (although it is typically yellow) and refractive index.
The composition of minerals, such as pumice, may be determined using a number of techniques. The composition of the mineral may be determined using an inductively coupled plasma (ICP) device, or by more conventional X-ray diffraction techniques. Thus, the level of crystallinity of a mineral, e.g., pumice, may be investigated and determined.
The clarity of a polyolefin film is extremely important in many applications. For example, in the food packaging industry, polyolefin films having high clarity are extremely desirable, so that the contents of a package may be readily inspected without disrupting the seal provided by the film around the product. The clarity of a film is typically quantified in terms of the amount of haze, measured using standard methods. One such method is ASTM D 1003-95, Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics, incorporated by reference herein in its entirety.
Accordingly, there remains a need for an antiblock agent which achieves high clarity when incorporated into polymer, e.g., polyolefin, films, while being economically feasible, readily available, which produces very low haze when incorporated into a film, which does not present the health and safety concerns of prior antiblock agents, and yet provides antiblock properties superior to prior antiblock agents.